The present invention relates to (i) a laser element doped with rare earth ions and an optical amplifier element doped with rare earth ions to be used in the fields of optical communication, optical information processing, light-applied measurements and control thereof and the like in which coherent light is applied, and (ii) a rare-earth-ion-doped short-wavelength laser light source apparatus to be used in the fields of optical information processing, optical measurements, environmental measurements and the like.
Since the 1960's, there have actively been conducted researches and developments on rare earth ions with which an optical material is to be doped, and proposed rare-earth-ion-doped solid-state laser elements which have a variety of arrangements and which oscillate lasers at a variety of wavelengths. Generally, these rare earth ions are excited by a pumping source comprising a flash lamp which has high output power. However, such a flash lamp also emits light at wavelengths which cannot be absorbed by the rare earth ions. This disadvantageously presents the problems that a rare-earth-ion-doped solid-state laser is lowered in output efficiency and that an adverse effect by heat cannot be avoided.
In 1980's, there has been used, as a pumping source, any of lasers and specially semiconductor lasers adapted to oscillate laser in an absorption area inherent in rare earth ions. As the output of such a semiconductor laser becomes higher, the rare-earth-ion-doped solid-state laser is improved in output efficiency and lowered in sizes and cost.
A rare-earth-ion-doped solid-state laser to be excited by a semiconductor laser is represented by a Nd-ion-doped YAG (Yittrium-Aluminum-Garnet) laser which emits light at 1.064 .mu.m.
FIG. 14 shows the arrangement of a conventional rare-earth-ion-doped solid-state laser. Shown in FIG. 14 are a solid-state laser element 601 comprising YLiF.sub.4, an incident portion 602 of the solid-state laser element 601, a light emitting portion 603 of the solid-state laser element 601, an incident lens 604, exciting light 605, laser 606 to be supplied, a pumping source 607 and a light emitting lens 608.
FIG. 15 shows the energy levels and energy transitions of Tm (thulium) ions with which the solid-state laser element 601 was doped. Shown in FIG. 15 are a level transition 651 due to light absorption, a nonradiative transition 652 due to phonon emission, a radiative transition 653 due to light emission and energy levels 654, 655, 656, 657. The ordinate 658 shows the energy in unit cm.sup.-1 (Kayser).
As shown in FIG. 14, the exciting light 605 at an optical wavelength of 795 nm oscillated from the pumping source 607, is focussed by the incident lens 604, and the exciting light 605 thus focussed is incident upon the solid-state laser element 601 through the incident portion 602. The Tm ions undergo a change in energy level as shown in FIG. 15. More specifically, upon absorption of the energy discharged from the exciting light 605, the Tm ions experience a transition from the ground state level 654 to the high energy level 656 and also experience the nonradiative transition 652 from the high energy level 656 to the first intermediate level 655. At the first intermediate level 655, the Tm ions emit light at 1.50 .mu.m and therefore experience a transition from the first intermediate level 655 to the second intermediate level 657. When the Tm ions experience a radiative transition from the first intermediate level 655 to the second intermediate level 657, the Tm ions emit light. The light thus emitted is caused to resonate by reflection films disposed at the incident portion 602 and the light emitting portion 603, and is then emitted, as laser at 1.5 .mu.m, from the light emitting portion 603.
Conventional solid-state laser elements are mainly of a 3- or 4-level system in which rare earth ions, for example Tm ions, with which an optical material was doped, are excited by exciting light to raise the energy level thereof, after which there is emitted light having an energy level lower than that of the exciting light, i.e., laser at a wavelength longer than that of the exciting light.
However, there are some types of rare earth ions which exhibit a so-called excited-state absorption (ESA) in which the fluorescence lifetime at a high energy level is long and in which the ions are again excited from the high energy level to a higher energy level when the ions are excited by a high output. This higher energy level is higher than the energy level of the exciting light. This enables laser to be oscillated at a wavelength shorter than that of the exciting light. Such excitation to a higher energy level is called upconversion excitation, and a short-wavelength laser element obtained in such a manner is called an upconversion laser element.
As examples of the upconversion laser element, there have been reported a laser element comprising a Ho ion- or Er ion-doped solid-state laser element and adapted to oscillate green light at a 530-nm band, and a laser element comprising a Tm ion- or Pr ion-doped solid-state laser element and adapted to oscillate blue light. These laser elements are discussed, for example, in "Generation of Visible Radiation by Upconversion" written by Aki SHIKIDA et al (Laser Research, Vol. 20, No. 4, 1992).
The upconversion laser element mainly experiences two absorption transitions and therefore requires, as exciting light, two lights at two wavelengths. As reported by Allain et al in "Blue Upconversion Fluorozirconate Fiber Laser", Electron Letters, Vol. 26, p.p. 166-167, 1990, a solid-state laser element doped with Tm ions for example, oscillates blue light at a wavelength of 450 nm to 480 nm because the Tm ions experience a transition of ground-state absorption (GSA) by light at 680 nm and then experience an excited-state absorption transition by light at 650 nm. In this case, a Kr laser is used as the pumping source.
The Kr laser is expensive and requires a large-scale device. Accordingly, when the Kr laser is used as the pumping source, a short-wavelength laser light source apparatus using the Kr laser as the pumping source is considerably limited in industrial applicability.
It has also been proposed an upconversion element in which Tm ions are excited by lights at three wavelengths to oscillate light at 480 nm. This is reported in "CW Room-Temperature Blue Upconversion Fibre Laser", S. G. Grubb, K. W. Bennett, R. S. Cannon and W. F. Humer Electronics Letters, Vol. 28, No. 13, pp 1243-1244. More specifically, there is used a Nd:YAG solid-state laser to be excited by a semiconductor laser and there are utilized three wavelengths of 1.112 .mu.m, 1.116 .mu.m and 1.123 .mu.m oscillated by the Nd:YAG solid-state laser, so that the Tm ions experience a ground-state absorption transition and two excited-state absorption transitions, thus achieving upconversion.
The following will discuss an upconversion laser element using Tm ions with reference to FIG. 16 showing energy levels of the Tm ions.
Shown in FIG. 16 are an absorption transition 661 of exciting light at 1.12 .mu.m oscillated by a pumping source, a transition 662 due to phonon emission, a radiative transition 663, and energy levels 664, 665, 666, 667, 668, 669, 670. The ordinate 671 shows the energy in unit cm.sup.-1 (Kayser).
The core of a fluoride-type optical fiber (2 m in length) is doped with 1,000 p.p.m of Tm ions (1 p.p.m refers to a part per million by weight). When exciting light at 1.12 .mu.m is incident upon this optical fiber, the Tm ions undergo the following change in energy level. Upon absorption of the exciting light, the Tm ions experience a transition from the ground state level 664 to the first high energy level 666. Then, the emission of phonon 662 causes the Tm ions to experience a transition to the first intermediate level 665. Then, upon absorption of the same exciting light, the Tm ions experience a transition from the first intermediate level 665 to the second high energy level 668. Then, the emission of phonon 662 causes the Tm ions to experience a transition to the second intermediate level 667. Then, upon absorption of the same exciting light, the Tm ions experience a transition from the second intermediate energy level 667 to the third energy level 669. When the Tm ions experience a transition from the third high energy level 669 to the ground state level 664, the Tm ions emit blue light at 480 nm. The light thus emitted from the Tm ions is caused to resonate by reflection films disposed at the incident portion and the light emitting portion of the optical fiber, and is then oscillated, as laser, from the light emitting portion.
Such conventional rare-earth-ion-doped laser elements present the following problems.
As a first problem, there is not available a high-output pumping source as the pumping source used in each of the conventional rare-earth-ion-doped laser elements for emitting exciting light at 1.12 .mu.m. That is, a usual Nd ion-doped solid-state laser oscillates laser at 1.064 .mu.m. This is because the transition probability of light at 1.064 .mu.m is twice as high as that of light at 1.12 .mu.m. To obtain oscillation of laser at 1.12 .mu.m, it is required to make a special contrivance on a solid-state laser element for oscillating laser at 1.064 .mu.m. For example, provision should be made such that light at a wavelength in the vicinity of 1.12 .mu.m is reflected from the reflection films of a resonator, or that an etalon having a wavelength selectivity is inserted in the resonator. When such provision is made, it is possible to oscillate laser at 1.12 .mu.m. However, the laser is lowered in output, causing the energy contributing to excitation to be insufficient.
As a second problem, to achieve upconversion, there are required at least two exciting lights including one exciting light for ground-state absorption and at least one exciting light for excited-state absorption. Generally, these two exciting lights are to be supplied from two pumping sources. A short-wavelength laser light source apparatus using two pumping sources, becomes inevitably large-scaled and expensive, and is therefore limited in industrial applicability.
As a third problem, to cause at least two lights at different wavelengths to be simultaneously incident upon an optical material doped with rare earth ions, it is required to provide a resonator with a multilayer reflection film mirror from which lights are reflected at a plurality of wavelengths, or to correct the chromatic aberration of the lens system resulting from a difference in wavelength between the exciting lights, or to dispose a complicated optical system. Accordingly, the short-wavelength laser light source apparatus is disadvantageously increased in the number of component elements and cost.
As a fourth problem, rare earth ions are excited by exciting lights at three wavelengths which comprise one wavelength of exciting light for ground-state absorption and two wavelengths of exciting lights for excited-state absorption, and of which absorption wavelength areas overlap one another. Accordingly, in the areas where absorption wavelengths overlap one another, the coefficients of ground-state absorption and excited-state absorption are low to lower the absorption efficiency.
As a fifth problem, there is noted a mode overlap between exciting light and oscillated light due to the cutoff wavelength of the optical fiber. More specifically, when using exciting light at 1.12 .mu.m which is infrared light, the core of the optical fiber must have such dimensions as to match the wavelength of the exciting light such that the optical fiber propagates the exciting light in a single transverse mode with the loss minimized. That is, it is required to set the cutoff wavelength .sub.c of the optical fiber to 0.9 to 1.0 .mu.m. However, when the core of the optical fiber is made in such large dimensions, the optical fiber propagates oscillated light at 480 nm in a multiple transverse mode. This considerably deteriorates the overlap of the propagation mode of the exciting light in the optical fiber on the propagation mode of the oscillated light in the optical fiber. This lowers the efficiency of the output of oscillated light with respect to the output of the exciting light.